Method of forming a negative differential resistance device

ABSTRACT

A negative differential resistance (NDR) field-effect transistor element is disclosed, formed on a silicon-based substrate using conventional MOS manufacturing operations. Methods for improving a variety of NDR characteristics for an NDR element, such as peak-to-valley ratio (PVR), NDR onset voltage (V NDR ) and related parameters are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/314,735 entitled “Method Of Forming A Negative DifferentialResistance Device”, filed Dec. 9, 2002 now U.S. Pat. No. 6,980,467, andis related to the following applications, which are hereby incorporatedby reference as if fully set forth herein: U.S. patent application Ser.No. 10/314,785, filed Dec. 9, 2002 entitled “Process for ControllingPerformance Characteristics of a Negative Differential Resistance (NDR)Device”; and U.S. patent application Ser. No. 10/321,031, filed Dec. 17,2002 entitled “Integrated Circuit Having Negative DifferentialResistance (NDR) Devices With Varied Peak-to-Valley Ratios (PVRs)”.

FIELD OF THE INVENTION

This invention is directed to processes for making semiconductordevices, circuits, dies and systems, particularly those that utilizenegative differential resistance (NDR) field-effect transistor (FET)elements.

BACKGROUND OF THE INVENTION

Silicon based devices that exhibit a negative differential resistance(NDR) characteristic have long been sought after in the history ofsemiconductor devices. A new type of CMOS compatible, NDR capable FET isdisclosed in the following King et al. applications:

Ser. No. 09/603,101 entitled “A CMOS-PROCESS COMPATIBLE, TUNABLE NDR(NEGATIVE DIFFERENTIAL RESISTANCE) DEVICE AND METHOD OF OPERATING SAME”;and

Ser. No. 09/603,102 entitled “CHARGE TRAPPING DEVICE AND METHOD FORIMPLEMENTING A TRANSISTOR HAVING A NEGATIVE DIFFERENTIAL RESISTANCEMODE” now issued as U.S. Pat. No. 6,479,862 on Nov. 12, 2002; and

Ser. No. 09/602,658 entitled “CMOS COMPATIBLE PROCESS FOR MAKING ATUNABLE NEGATIVE DIFFERENTIAL RESISTANCE (NDR) DEVICE”; all of whichwere filed Jun. 22, 2000 and which are hereby incorporated by referenceas if fully set forth herein. The advantages of such device are well setout in such materials, and are not repeated here.

As also explained in such references, NDR devices can be used in anumber of circuit applications, including multiple-valued logiccircuits, static memory (SRAM) cells, latches, and oscillators to name afew. The aforementioned King et al. applications describe abreak-through advancement that allows NDR devices to be implemented insilicon-based IC technology, using conventional planar processingtechniques as for complementary metal-oxide-semiconductor (CMOS) FETdevices. The integration of NDR devices with CMOS devices provides anumber of benefits for high-density logic and memory circuits.

It is clear, from the advantages presented by the above-described NDRdevice, that overall improvements in manufacturing, testing andoperation of the same are desirable to refine and proliferate suchtechnologies.

In addition, enhancements in trap location control, trap energy levelcontrol, and trap formation, are also useful for these types of NDRdevices, and could be beneficial to other types of trap-based devices aswell.

Furthermore, the prior art to date has been limited generally to devicesin which the peak-to-valley ratio (PVR) is not easily adjustable. Itwould be useful, for example, to be able to control the PVR directlyduring manufacture, so as to permit a wide variety of NDR behaviors fordifferent circuits on a single die/wafer. Alternatively, the ability tocontrol PVR during normal operation of a device would also be useful,but is generally not possible with current NDR technologies.

SUMMARY OF THE INVENTION

The object of the present invention, therefore, is to address theaforementioned limitations in the prior art, and to provide additionalembodiments of trapping devices, NDR devices, and methods of making,operating and testing the same. These and other objects are accomplishedby various embodiments of the present invention as described in detailbelow, it being understood by those skilled in the art that manyembodiments of the invention will not use or require all aspects of theinvention as described herein.

A first aspect of the invention, therefore, concerns a method of forminga silicon based negative differential resistance (NDR) field effecttransistor (FET) comprising the preferred steps of: providing asubstrate; forming a first NDR region for the NDR FET over a firstportion of the substrate using a first impurity, the first NDR regionbeing adapted for imparting an NDR characteristic to the NDR FET;placing a second impurity in the first portion of the substrate toadjust a threshold voltage characteristic of the NDR FET; performing afirst thermal treatment operation for the NDR FET after the above arecompleted; forming a gate insulating layer for the NDR FET over thefirst portion of the substrate; performing a second thermal treatmentoperation for the NDR FET; forming a gate electrode for the NDR FET;forming a source region and a separate drain region for the NDR FETadjacent to the gate electrode, the source region and drain region beingcoupled through an NDR FET channel located in the first portion of thesubstrate.

In this manner, an NDR FET preferably operates with a negativedifferential resistance characteristic when sufficient charge carriersfrom the channel are temporarily trapped in the first NDR region. Thefirst impurity is preferably a first type dopant, and the secondimpurity is preferably a second type dopant, which is opposite to thefirst type dopant. The first thermal treatment operation is preferablyperformed with a furnace, while the second thermal treatment operationis preferably performed with a rapid thermal anneal system. Furthermore,in addition to the above, a third thermal treatment operation ispreferably performed after the gate electrode is formed.

In later steps, a silicide contact to the gate electrode and/or one orboth of the source region and the drain region can be formed.

Some embodiments of the invention, therefore, are silicon based negativedifferential resistance (NDR) field effect transistor (FET) which have apeak-to-valley current ratio (PVR) that exceeds ten (10) over atemperature range of 50° C. In some instances, a PVR can exceed onethousand (1000) over a temperature range of 100° C.

In other embodiments, a silicon on insulator (SOI) substrate is used; avariety of substrates are suitable for the present invention, includingsilicon carbide (SiC) or strained Si.

The impurities added to the FET are used as charge trapping sites, whichpreferably have an energy characteristic that is higher than aconduction band edge of the substrate.

In other embodiments, an NDR FET and a non-NDR FET are made at the sametime using common manufacturing operations. The non-NDR FET is formed ina second region of the semiconductor substrate. For example, isolationregions, LDD implants, gate insulators, gate electrodes, contacts,source/drain implants, etc., can be done using a common processing step.In such instances, an NDR region for an NDR device is preferablyconstructed from a gate insulator region for an NDR FET.

In still other embodiments, two different types of NDR devices can beformed in a common substrate. Thus, a second NDR region for another NDRelement is formed over a third region of the semiconductor substrate,the second NDR region being adapted for imparting a second NDRcharacteristic different from an NDR characteristic for a first NDR FET.

A related aspect of the invention, therefore, pertains to a charge-trapbased negative differential resistance (NDR) element, which operateswith an NDR characteristic defined by a peak current and a valleycurrent. By appropriate distribution of charge traps in a trappingregion of the NDR element, including controlling a concentration andenergy of the same, a peak-to-valley current ratio (PVR) for the NDRelement can be imparted which exceeds ten (10) over a temperature rangespanning 50° C.

In other embodiments the PVR can be constructed to vary by less than afactor of five in an operating temperature spanning 25° C. and 125° C.In still other embodiments, the PVR exceeds 1000 in an operatingtemperature spanning 25° and 125°. The trapping region preferably formsan interface with a channel of a field effect transistor associated withthe NDR element.

Other embodiments of charge trapping devices can be similarlyconstructed to achieve similar performance.

Another aspect of the invention concerns a method of forming a negativedifferential resistance (NDR) device comprising the steps of: forming agated silicon-based NDR element; and setting a peak-to-valley ratio(PVR) characteristic of the gated silicon-based NDR element duringmanufacture of the silicon-based semiconductor transistor to a targetPVR value located in a range between a first PVR value and a second PVRvalue. Thus, a target PVR value can be varied during manufacturing ofthe NDR device within a semiconductor process such that the NDR devicecan be configured to have a PVR value ranging between a first usable PVRvalue and a second usable PVR value, where the first usable PVR valueand the second PVR value vary by at least a factor of ten (10).

In some instances, a desired PVR value can be set using a singleprocessing operation, such as an implant.

A preferred approach uses only metal oxide semiconductor (MOS)compatible processing operations. The inventive process is flexibleenough so that within a particular manufacturing facility, a firstsemiconductor substrate on a first wafer and a second separatesemiconductor substrate on a second wafer can have different target PVRvalues imparted at different times. The different PVR values can beprogrammed into a semiconductor processing apparatus such as animplanter, a furnace, an anneal chamber, a deposition reactor, etc. AnNDR voltage onset point (VNDR) is also preferably set duringmanufacture.

In still other more specific embodiments, a PVR (and/or a VNDR) valuecan be set during manufacture by controlling one or more general processparameters.

For example, in some embodiments, a PVR and/or VNDR can be set duringmanufacture by controlling a thickness of a gate insulator grown for theNDR device. In particular, a PVR characteristic can be increased simplyby increasing a thickness of the gate insulator. The gate insulator ispreferably at least 5 nm thick, and can be a single layer, or acomposite of two different materials. In some applications the gate willinclude both a thermal oxide and a deposited oxide based material. Thus,it is possible in some applications to have a common substrate thatincludes a silicon based NDR device with a first PVR characteristicusing a first gate insulator thickness and a second silicon based NDRdevice with a second PVR characteristic using a second gate insulatorthickness.

In another embodiment, a PVR and/or VNDR can be set during manufactureby controlling a channel length used for a silicon based NDR FET.Because the present invention scales very well, a PVR characteristictracks a channel length, so that a higher PVR can be achieved by using asmaller channel, and a lower PVR can be achieved by using a longerchannel. Accordingly, PVR characteristics can be established throughconventional masking operations which define a channel length, and/orwhich define a source/drain region implant. The channel can also have asize that is defined by a variable sized spacer formed on the sidewallsof a gate electrode. Thus, a PVR value can be increased significantlythrough even small reductions in channel lengths.

In still another embodiment, a PVR and/or VNDR can be set duringmanufacture by controlling an impurity species and/or impurity doseintroduced into a charge trapping layer associated with the NDR elementto match a target charge trap profile. In a preferred approach Boron isselected as the impurity at a dose ranging from 1*10¹⁴/cm² to 3*10¹⁴atoms/cm² and an energy of ≦10 keV. This results in a target charge trapprofile in which a concentration of charge traps is greater than about1*10¹⁹ atoms/cm³ at a trapping region of the charge trapping layer, andless than about 1*10¹⁸ atoms/cm³ at a bulk region of the charge trappinglayer. A PVR can thus be altered merely by selecting another impurity,another dosage, etc. For example, increasing an impurity dose of Boronby 50% can result in an increase of greater than 100% in a PVRcharacteristic. As with the other PVR processing embodiments, an NDRvoltage onset point (VNDR) can also be controlled in this fashion.

In still another embodiment, a PVR and/or VNDR can be set duringmanufacture by controlling an overall trap distribution, such as atarget location of the charge traps and a target concentration of thecharge traps. In a preferred embodiment, the charge traps aredistributed within a target location is a region that is less than about0.5 nm thick. Furthermore, a concentration of traps is arranged so thatan interface concentration is least an order of magnitude greater thanin bulk areas of the charge trapping layer.

In other embodiments, a PVR and/or VNDR can be set during manufacture bycontrolling a rapid thermal anneal (RTA) operation. A preferred approachis to use a short cycle at a temperature that exceeds 1000° C. for atleast part of the cycle in a conventional lamp based chamber. This typeof operation serves to focus and concentrate charge traps at a channelinterface region, as opposed to bulk regions.

In still other embodiments, a PVR and/or VNDR can be set duringmanufacture by controlling a lightly doped drain operation, including animplant species and/or dosage, performed during formation of a lightlydoped drain region operation. In a preferred embodiment, arsenic is usedas the dopant species at a dosage in excess of 1*10¹⁵ atoms/cm² toeffectuate the implant operation. In other embodiments, phosphorus isused as the dopant species at a dosage in excess of 1*10¹⁵ atoms/cm² toeffectuate the implant operation. Since Arsenic achieves a PVR that isat least 2 times greater than Phosphorus, it is preferred for thoseapplications where PVR is more critical to the operation of a circuit.

Related aspects of the invention concern a semiconductor processingapparatus for manufacturing a negative differential resistance (NDR)device on a silicon wafer which can be programmed to tailor a specificPVR value on a wafer-by-wafer basis (or even die by die). The apparatusis preferably located in a conventional semiconductor fab, and includesa programmable controller responsive to a negative differentialresistance (NDR) related process recipe associated with making the NDRdevice. An NDR related process recipe includes one or more processingsteps associated with effectuating a target peak-to-valley current ratio(PVR) for an NDR device. The processing chamber coupled to theprogrammable controller is configured to perform at least onesemiconductor processing operation on the silicon wafer based on the NDRrelated process recipe. The semiconductor processing operation can bevaried within the processing chamber to achieve a PVR value that variesbetween a first value, and a second value that is at least twice thefirst value.

In other embodiments, the PVR value can be varied between 10 and 100 inthe semiconductor processing apparatus. The process chamber can be animplanter, an RTA chamber, a deposition reactor etc.

Other aspects of the invention concern different types of optimizationsfor charge trap profiling for charge trap devices, including NDRdevices.

In an NDR FET embodiment, counter-doping is performed to improve athreshold voltage. Thus, a semiconductor device having a control gate, asource region, and a drain region is formed using the steps of:providing a substrate having a first type of conductivity; forming achannel between the source and drain region for carrying the chargecarriers between the source and drain regions; the channel is doped intwo separate operations such that: during a first channel dopingoperation the channel is doped with first channel impurities that alsohave the first type of conductivity; during a second channel dopingoperation the channel is counter-doped with second channel impuritiesthat have a second type of conductivity. The second type of conductivityis opposite to the first type of conductivity. As a result of the firstchannel doping operation and the second channel doping operation thechannel region as formed has a net first type of conductivity. A chargetrapping region that has an interface with the channel is also formed.The charge trapping region has charge trapping sites, which temporarilytrap charge carriers along the interface and permit the device tooperate with a negative differential resistance characteristic. Thecharge trapping sites are derived at least in part from the firstchannel impurities forming a charge trap distribution that issubstantially concentrated at the interface.

In a preferred embodiment, Arsenic is used for the second channel dopingoperation, while Boron is used for the first channel doping operation.While silicon is used as a preferred substrate, other substrates couldbe used, such as SOI, SiC, strained Si, etc. Moreover, different crystalorientation variants of silicon (111, 100, 110) may result in differentcharge trapping characteristics.

The charge trapping region is typically formed as part of gate insulatorfor the semiconductor device. In other variations, the charge traps canbe directly implanted through a gate insulator after the latter iscompleted. In still further variants, the charge traps can be formed aspart of a two layer trapping region, such as would be derived from acombined thermal oxide and deposited oxide.

In other variations, the charge trapping region can be engineered to notextend throughout an entire length of the interface with the channel. Inother instances, the charge trapping region extends from a source regionto enhance source side trapping. In still other embodiments, trappingsites are distributed unevenly along the interface to effectuate avariable trapping rate for the energetic carries along the interface. Atrapping rate can also be controlled in some instances, so that itvaries substantially proportional to a distance along the interface,and/or is preferentially greater in one region over another—i.e., suchthat in a source region it is greater than that near a drain region.

In other embodiments, the charge trapping sites are formed in twodistinct operations. For example, an implant operation is used forforming a first set of charge trapping sites, and a heat treatmentoperation (such as in an steam ambient) forms a second set of chargetrapping sites. In still other embodiments, different implants could beused of the same species, or different atomic species to createdifferent types of charge traps (i.e., such as Boron and silicon ormetal nanoparticles).

A further related aspect of the invention concerns using annealingoperations to help ensure that impurities are preferentiallyconcentrated at an interface, where they can form appropriate trapsites. This is achieved by forming a silicon based negative differentialresistance (NDR) semiconductor device with the steps of: providing asubstrate; and forming a channel region for carrying a current of chargecarriers for the silicon based NDR semiconductor device; and implantingfirst impurities into the channel region; and forming a first dielectriclayer that has an interface with the channel; and annealing the channelregion to reduce implantation defects and distribute the firstimpurities so as to concentrate them along the interface with thechannel. The first impurities as distributed along the interface formcharge trapping sites with an energy level adapted for temporarilytrapping the charge carriers to effectuate an NDR characteristic.

In a preferred embodiment, the first impurities have a firstconductivity (p) type that is the same as the substrate. The siliconbased NDR semiconductor device is typically a field effect transistor(FET), but can include other charge trap based NDR devices.

In still another variant, additional annealing operations can beperformed to further enhance a trap distribution. Thus, thisimplementation involves performing a plurality of separate annealingoperations on the semiconductor structure, wherein at least a first oneof the separate annealing operations is adapted so as to distribute andconcentrate the carrier trapping sites along an interface with thetransistor channel region and with a reduced concentration in a bulkregion of the trapping layer. Later separate annealing operations areadapted to alter a concentration and/or arrangement of the chargetrapping sites along the interface.

A further related aspect, therefore, concerns a silicon based fieldeffect transistor (FET) comprising a trapping layer proximate to atransistor channel region for the FET, the trapping layer including acarrier trapping sites configured for trapping and de-trapping carriersfrom the channel region. The carrier trapping sites are distributed suchthat a concentration of the carrier trapping sites in a bulk region ofthe trapping layer is at least one order of magnitude less than it isalong an interface with the transistor channel region. In this fashion,the FET can exhibit negative differential resistance as a result of thetrapping and de-trapping of carriers.

In a preferred embodiment, a concentration of the carrier trapping sitesat the interface per cubic centimeter is at least two orders ofmagnitude greater than a concentration of the carrier trapping siteswithin the bulk region of the trapping layer. Furthermore, theconcentration of an impurity per cubic centimeter used for the carriertrapping sites is at least two times higher at a trapping layer-channelinterface than in the channel region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a preferred embodiment of a negativedifferential resistance (NDR) field effect transistor (FET) of thepresent invention;

FIG. 2 is a representative plot of the current-vs.-voltagecharacteristic of the NDR FET of FIG. 1;

FIG. 3A is a band diagram generally illustrating the energy relationshipof conduction bands, valence bands and charge trapping sites of a chargetrapping region, including variants which can be used in a preferredembodiment of the present invention;

FIG. 3B is a plot of the impurity concentration vs. depth in oneembodiment of the NDR FET;

FIG. 4 depicts the overall steps used in a preferred process to make NDRdevices, including an NDR FET of the present invention;

FIGS. 5 to 16 generally illustrate the steps used in a preferredembodiment of an NDR device manufacturing process of the presentinvention.

FIG. 5 shows a schematic cross-sectional view of a starting substrateused to manufacture an NDR element—including a preferred NDR FETembodiment of the present invention—as well as other conventionalsemiconductor elements and devices;

FIG. 6 is a schematic cross-sectional view showing the step of formingelectrically isolated active areas in surface regions of the substrate;

FIG. 7 is a schematic cross-sectional view showing the step of forming asacrificial insulating layer on the surface of the substrate in an areawhere an NDR FET of a preferred embodiment is to be formed;

FIG. 8 is a schematic cross-sectional view showing the step ofselectively introducing a first type of impurities into the surface ofthe substrate in an area where an NDR-FET of a preferred embodiment isto be formed;

FIG. 9 is a schematic cross-sectional view showing the step ofselectively introducing a second type of impurities into the surface ofthe substrate in an area where an NDR-FET of a preferred embodiment isto be formed, as part of a counter-doping step;

FIG. 10 is a schematic cross-sectional view showing the step of formingan additional insulating layer on various regions of the surface of thesubstrate where active devices, including NDR FET and other conventionalFETs, are to be formed;

FIG. 11 is a schematic cross-sectional view showing the step ofdepositing a gate film for both NDR FETs and conventional FETs;

FIG. 12 is a schematic cross-sectional view showing the step ofpatterning the gate film into gate electrodes for both NDR FETs andconventional FETs;

FIG. 13 is a schematic cross-sectional view showing the effects of oneor more post-gate oxidation anneal steps used to increase a density ofcharge traps at a channel interface of the NDR FET of a preferredembodiment;

FIG. 14 is a schematic cross-sectional view showing the step of formingsource and drain extension regions with an Arsenic implant;

FIG. 15 is a schematic cross-sectional view showing the step of formingmore heavily doped source/drain contact regions for an NDR FET and otherconventional FETs;

FIG. 16 is a schematic cross-sectional view showing the final results ofdepositing an electrically insulating interlayer film, forming contactholes in the interlayer film, and depositing a metal layer andpatterning the metal layer to form interconnections to the NDR-FET andconventional FETs.

FIGS. 17A–17K are charts, graphs and other depictions of experimentaldata obtained for various embodiments of an NDR FET device.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described with referenceto the Figures provided herein. It will be appreciated by those skilledin the art that the present examples are but one of many possibleimplementations of the present teachings, and therefore the presentinvention is not limited by such.

The present invention is expected to find substantial uses in the fieldof integrated circuit electronics as an additional fundamental “buildingblock” for digital memory, digital logic, and analog circuits. Thus, itcan be included within a memory cell, within a Boolean function unit,and similar such environments.

Brief Summary of Prior Art

FIG. 1 shows a prior art NDR FET 100 of the type described in the Kinget al. applications noted earlier. This device is essentially a siliconbased MISFET that includes an NDR characteristic as well. Thus, thefeatures of device 100 are created with conventional MOS based FETprocessing, modified where appropriate as to include operations foreffectuating an NDR behavior.

Accordingly, in FIG. 1, a gate electrode 110 is coupled to a gateterminal 115 for receiving a gate select signal. The device 100 isformed within a substrate 120 (preferably p-type) and includes awell-known source 140 and drain region 150 coupled by a channel 135. Abody contact terminal 125 provides a body bias to device 100, andsource/drain voltages are provided through conventional source/drainterminals 145 and 155 respectively. A gate insulator layer 130 issituated between channel 135 and gate electrode 110. Again, thesefeatures are all common to most standard MISFETs; additionalconventional features (such as retrograde substrate doping, “halo” or“pocket” doping, gate-sidewall spacers, shallow source and drainjunctions) are not shown for purposes of better illustrating the natureof the invention.

The additional features in device 100 which are somewhat different froma conventional FET and which impart an NDR behavior include thefollowing: (1) a slightly thicker gate electrode 130; (2) a lightlyp-type doped channel surface region; and (3) a charge trapping region137. These modifications cooperate to impart an NDR behavior to such FETfor reasons set out in detail in the aforementioned King et al.applications.

This behavior is illustrated in FIG. 2, where device drain currentversus drain voltage is plotted for two different gate voltages to showhow an NDR mode can be affected by a suitable selection of a gatevoltage as well. It can be seen that for a fixed gate voltage V_(GS)relative to the source, a drain current I_(DS) firstly increases in afirst region 210 with drain-to-source voltage V_(DS), similarly to thebehavior that is seen in drain current in a conventional n-channel MOStransistor. However, in region 220, beyond a certain drain voltagelevel, drain current decreases with further increases in voltage, i.e.the device exhibits an NDR mode with NDR characteristics. The drainvoltage at which the drain current begins to decrease (i.e., point 225where V_(DS)=V_(NDR)) is adjustable through suitable selections ofimpurity species, channel length, threshold voltage, etc.

As seen also in FIG. 2, the invention can be seen as exploiting the factthat, as the threshold voltage V_(t) dynamically increases (because ofthe accumulation of trapped charges) with increasing drain-to-sourcevoltage V_(DS), a drain current I_(DS) (which is proportional toV_(g)−V_(t)) will decrease. Thus, a current value depicted in curve 228will generally follow the set of continuous curves 229 shown in FIG. 2for a given V_(g) and varying V_(t). Unlike other prior art devices, theso-called “peak-to-valley ratio,” a key figure of merit in NDR devices,as well as the NDR onset voltage, can also be precisely tuned throughsuitable combinations of impurity species, doping concentrations, devicegeometries and applied voltages. Furthermore, an NDR behavior of thepresent invention can achieve a PVR well in excess of 100, 1000, or even10⁶ across a wide temperature range (−40° C. to +150° C.), which farexceeds the capabilities of conventional NDR devices.

It will be appreciated by those skilled in the art that the entirety ofthe preceding description is merely provided by way of background tobetter illustrate the context of the present inventions, and thus, bynecessity, is somewhat abbreviated. It is not intended to be, nor shouldit be taken, as a complete analysis of the structural, operational orphysical of the aforementioned King et al. inventions. Nor should it inany way be construed as limiting in any way of the inventions disclosedtherein.

Tray Energy Characteristics

FIG. 3A illustrates a preferred energy-band diagram (electron energy vs.distance in the direction perpendicular to a semiconductor surface) ofdevice 100 depicted in FIG. 1. When a gate bias is applied, an inversionlayer of electrons is formed at the semiconductor surface, i.e. the FETis turned on. A gate 310 is shown to be heavily dopedpolycrystalline-silicon (poly-Si), a gate dielectric 330 is shown to beSiO₂, and a silicon semiconductor substrate 320 is p-type as is the casein modem CMOS technologies. Again, it will be understood that othermaterials known in the art can be substituted instead.

A lower edge E_(c) of the conduction band of allowed electron energystates for semiconductor material 320 is shown, as well as an upper edgeE_(v) of a valence band of allowed electron energy states. Conventionaldevice physics theories mandates that there are no allowed electronenergy states within a band gap corresponding to a range of energiesfrom E_(v) to E_(c). Accordingly, no mobile electron in semiconductormaterial 320 can have an energy within this range.

As seen in FIG. 3A, a conduction-band electron in a channel region (nearan interface of gate dielectric 330 and semiconductor substrate 320 mustlose energy (e.g. via a lattice collision) in order to become trapped bya first type of charge trap 336 which has an energy level below E_(c).Afterwards, it must be supplied with energy (e.g. from latticevibrations) in order to be detrapped back into the conduction band insilicon semiconductor substrate 320. For reasons, which are apparentfrom the aforementioned King et al. applications, this type of trap,therefore, is not particularly useful for effectuating an NDRcharacteristic.

In contrast, a second type of charge trap 335, which has an energy levelvery near but above E_(c) can trap a conduction-band electron with totalenergy equal to its energy level, without requiring a lattice collision.Of course, charge trap 335 has an additional benefit in that it can alsotrap conduction-band electrons which have energies higher than an energylevel of such trap. For these second type of traps, a trapped electroncan easily move back into an allowed energy state in the conductionband, and hence it is easily detrapped. These second types of traps areparticularly suited for adapting a conventional FET to operate with anNDR characteristic. In will be noted that interface traps which areenergetically located well above the semiconductor conduction band edge(not shown) will have no effect on FET performance until a significantpercentage of the mobile carriers in the channel have sufficient kineticenergy to become trapped.

Thus, a preferred primary mechanism for achieving NDR behavior in aninsulated gate field-effect transistor is to trap energetic (“hot”)carriers from a channel with traps that also rapidly de-trap. The trapsshould be configured preferably so that a trap energy level should behigher than the semiconductor conduction band edge, in order for it toprimarily (if not exclusively) trap hot carriers. For example, a trapwhich is energetically located 0.5 eV above the semiconductor conductionband edge can only trap electrons from the semiconductor which havekinetic energy equal to or greater than 0.5 eV. For high-speed NDR FEToperation, it is desirable to have the carrier trapping and de-trappingprocesses occur as quickly as possible, as this permits a rapid anddynamic change in a threshold voltage for the FET.

Thus, the King et al. NDR device uses tunneling to a charge trap, andnot tunneling to a conduction band per se as required in someconventional NDR devices such as tunnel diodes. All that is required isthat the carriers be given sufficient energy to become trapped inlocalized allowed energy states within one or more dielectric layers(including for example a gate insulator layer made up of conventionaldielectric materials). It is not necessary to set up a complicated setof precisely tuned layers in a particular fashion to achieve acontinuous set of conduction bands as required in conventional NDRdevices, and this is another reason why such invention is expected toachieve more widespread use than competing technologies.

Finally, the physical distribution of such traps is also described inthe King et al. applications, and an approximate illustration of thesame is shown in FIG. 3B. This chart illustrates a general relationshipbetween charge trap concentration and distance. The left side of thegraph represents a bulk region of a trapping layer (in this case gatedielectric 330), which as can be seen, preferably has a very lowconcentration of charge traps (i.e., something less than 10¹⁶ atoms/cm³.The concentration increases rapidly near an interface 360, and thelatter contains a maximum concentration of impurities (in this caseBoron as noted with circles) useable as charge traps, preferably inexcess of 10¹⁹ atoms/cm³, and most preferably in excess of 10²⁰atoms/cm³. The concentration of Boron then decreases and is less againon a substrate side 320. The areal concentration of traps should not beexcessive, however (i.e., greater than 10¹⁴/cm²), because this can alsolead to undesirable electron conduction between the source and drain viatrap-to-trap hopping or channel-to-trap-to-channel hopping.

As can be seen in FIG. 3B, a majority of the charge traps should beplaced in close proximity to the channel, i.e. within 0.5 nm to 1.5 nmof the gate-dielectric/semiconductor interface, or right at theinterface itself. This can be achieved using a low-energy implant ofBoron at approximate dose of 2 to 3*10¹⁴ atoms/cm². It will beunderstood by those skilled in the art that the above figures are merelyrepresentative, and it is expected that such values (distances,concentrations) will vary in accordance with a particular processgeometry, device operational requirements, etc. Thus, the presentinvention is not limited to any particular arrangement of such traps. Atriangle symbol designates an overall “net” p-type doping in thechannel, which, for reasons set forth herein, should not be too highlydoped p-type, as this will undesirably increase a threshold voltage.

In operation, a trapping/de-trapping mechanism preferably starts at adrain end of the channel, and proceeds towards a source side of thechannel, to rapidly shut off the transistor. This is a result of thefact that the electrons have a maximum kinetic energy by the time theyreach the drain side of the channel, and thus are more likely to betrapped first in that region. As the voltage on the drain increases pastV_(NDR), the electrons will acquire more and more energy as a result ofthe increased field, at locations closer to the source. It can be seenfrom this mechanism as well that the NDR FET has good scalingcapabilities, because as a channel length shortens, thetrapping/detrapping mechanism can “switch” the transistor off even morerapidly.

This extra degree of freedom—i.e. the ability to independently control aFET channel conductivity through a source/drain bias voltage (inadditional to the conventional gate voltage modulation) provides yetanother example of the advantages presented by the present invention.Furthermore, this particular channel shut-off mechanism scales as wellor better than conventional MOSFET turn-off techniques, which, as iswell known, must rely on thinner and thinner oxides (or esotericmaterials) to achieve a sufficiently large field to deplete the channelof carriers in the conventional fashion (i.e., through an applied gatevoltage).

Overview of Process Flow

A preferred process flow for manufacturing an NDR device that isintegrable into a conventional MOS manufacturing process is illustratedin FIG. 4. The advantage of such process, as described in earlierapplications assigned to the applicant, is that additional conventionalnon-NDR circuitry (memory and logic) can thus be manufactured at thesame time.

Thus, as shown in FIG. 4, an initial substrate is chosen at step 405,which in a preferred embodiment is silicon, but which could be silicongermanium, silicon on insulator, strained silicon, silicon carbide, orany other desired material. Of course it will be understood that if anon-silicon substrate were selected to implement the present inventions,many of the processing steps below would have to be modified inaccordance with well-understood principles known to skilled artisans inthis field of art.

At step 410 isolation regions are formed in the substrate, which, in apreferred approach are shallow trench isolation (STI) regions. At step415, a sacrificial oxide layer is grown. At step 420, P wells and Nwells are formed in the substrate as well.

At step 425, impurities are introduced into NDR device regions, designedto facilitate a trapping/de-trapping mechanism noted earlier. Again, avariety of techniques are available for doing this as referenced in theaforementioned King et al. applications, including, for example, arelatively high dose implantation of boron (in excess of 1*10¹⁴atoms/cm²) into channel regions of NDR FETs.

At step 430, an optional NDR channel counter-doping step (n-type dopantimplant) is performed, to counter some of the effects of a NDR trapimplant, and thus reduce a net p-type channel doping concentration. Thisresults in lowered voltage thresholds, a steeper subthreshold swing, andcorrespondingly higher PVR values.

At step 435, an optional thermal anneal is performed, to remove damageto the semiconductor crystal lattice and thereby ensure properdistribution and concentration of the traps within a trapping regionstep. This is done to ensure that the traps do not migrate too far intothe trapping region, causing excessive leakage, slow operation, and poorreliability.

At step 440, the sacrificial oxide layer is optionally selectivelyremoved and a gate insulator is formed which can be used for both an NDRFET and a regular FET. This insulator can be comprised of multiplelayers of dielectric materials, and can be of different thickness andcomposition in an NDR FET region than in a regular FET region.

At step 445, an optional thermal anneal is performed (preferably a rapidthermal anneal, or “RTA”), to increase a density of charge traps at achannel/insulator interface.

At step 450, a gate electrode is formed which, again, can be used forboth an NDR FET and a regular FET.

At step 455, an optional post gate-etch re-oxidation anneal is performedto further modify (if needed) a distribution and density of charge trapsat a channel/insulator interface and/or to heal the gate insulator inthe regions along the edges of the gate electrodes.

At step 460, a “lightly doped drain” (LDD) implant is performed to formshallow source and drain regions (which can be for either/both NDR andnon-NDR FETs).

At step 465, an optional anneal is performed to repair any damage to thesemiconductor crystal lattice caused by the LDD implant.

At step 470, spacers are formed (which can be for either/both NDR andnon-NDR FETs) along the sidewalls of the gate electrodes to offset thedeep source/drain contact regions.

At step 471, optional raised source and drain contact regions areformed, preferably by selective epitaxial growth of silicon or asilicon-germanium alloy, which can be for either/both NDR and non-NDRFETs.

At step 475, a high-dose source/drain implant step is performed to formheavily doped source/drain contact regions, which, again, can be foreither/both NDR and non-NDR FETs.

At step 480, an anneal is performed to repair any damage caused by thesource/drain implant and to activate the implanted dopant atoms.

At step 485, an optional silicidation process module is used to form lowresistance contacts as required at gate and/or source/drainregions—again, for either/both NDR and non-NDR FETs.

At step 490, an electrically insulating passivation layer is depositedand holes are formed within this layer to allow electrical contact toregions of either/both NDR and non-NDR FETs.

At step 495, electrical interconnections (which can be made usingcopper, aluminum, or other low resistivity material) are formed over theNDR and non-NDR FETs to complete wiring of the devices and form anintegrated circuit. Such interconnections can be formed with multiplelayers of conductive material separated from each other by interposinginsulating layers with holes (“vias”) to allow for selective electricalconnection between layers.

Final passivation layers are then typically added in the back end of themanufacturing process as well.

A further detailed description now follows for those steps above whichare more germane to the present invention. As many of these steps areconventional, however, they are not explained herein in detail. Many ofthe particular structures, and formation steps for these layers andregions will depend on desired performance characteristics and processrequirements, and thus a variety of techniques are expected to besuitable. Furthermore, while examples of various techniques arepresented herein for a manufacturing process embodying the presentinvention, it will be understood by those skilled in the art that theseare merely exemplary of current state of the art approaches. Thus, thepresent invention is intended to encompass other yet-to-be developedprocesses currently unknown to the inventor over time that may replacesuch techniques and yet still be entirely suitable for use with thepresent invention.

Details of Process Flow

FIGS. 5 to 16 generally illustrate the detailed operational steps usedin a preferred embodiment of an NDR device manufacturing process of thepresent invention.

In particular, FIG. 5 shows a schematic cross-sectional view of astarting substrate used to manufacture an NDR element (in accordancewith step 405 described earlier)—including a preferred NDR FETembodiment of the present invention—as well as other conventionalsemiconductor elements and devices. As seen in FIG. 5, a preferredsubstrate 1000 consisting substantially of silicon (Si) is prepared.Because the NDR-FET and IGFET are n-channel devices, the portions of thesubstrate in which the NDR-FET(s) and IGFET(s) are to be formed arepreferably p-type.

In this regard it will be understood that starting substrate 1000 inFIG. 5 could also refer to a p-type well formed in the surface (withinthe top 1000 nm) of a starting substrate by ion implantation and/ordiffusion, either before or after the definition of “active” areas, inany number of known techniques known to those skilled in the art. Itshould be noted that substrate 1000 could also be silicon-on-insulator(SOI), and may eventually contain one or more additional layers ofsilicon-germanium alloy material or silicon carbide material (notshown). When selecting these latter substrates, of course, those skilledin the art will appreciate that the later processing steps describedbelow would have to be modified in well-known ways to accommodate suchchange.

FIG. 6 is a schematic cross-sectional view showing the step of formingelectrically isolated active areas in surface regions of the substrate(in accordance with step 410 described earlier) including in a firstarea 1015 where an NDR element (such as an NDR FET) is to be formed, anda second area 1015′ where a non-NDR element (such as a conventional FET)is to be formed. To better emphasize the present teachings, in FIG. 6(and other figures below) the later processing steps are shown in a“split” view to help explain the different impact and result on NDRregions and non-NDR regions across the substrate 1000 for variousoperational steps described herein. It will be understood by thoseskilled in the art that these figures are not intended to be to scale,and that actual substrate profiles will likely deviate (perhapssignificantly) in an actual manufacturing embodiment. Nonetheless theyare helpful to understand the important aspects of the presentinvention.

Consequently, in FIG. 6, electrically isolated “field” areas 1010 in asurface of substrate 1000 are formed by any of several currentwell-established techniques, including local oxidation of silicon(LOCOS) and/or shallow trench isolation (STI). The thickness of anisolation oxide layer 1010 typically falls in a range from 100 nm to 700nm, while a depth of shallow trench isolation structures typically fallsin the range from 100 nm to 1000 nm. Other later developed techniqueswill be useable with the present invention as well.

Moreover, it should be noted that the precise details of these areas arenot critical to the operation of the present invention, but asignificant advantage of course lies in the fact that such structures(however formed) can be share by both conventional active devices aswell as the NDR devices in accordance with the present teachings. Ofcourse, in some applications it may not be necessary to use such typesof isolation regions, and the present invention is by no means limitedto embodiments which include the same.

A sacrificial oxide layer 1018 is then grown. It will be understood byskilled artisans that since steps 415 and 420 are conventional and notmaterial to the present teachings, that consequently, they are notexplained in detail herein. Additional conventional processing steps(threshold adjusts for example, other insulating layers, or etch stoplayers, or plasma/heat treatments) that are incidental to the presentteachings are also omitted to better explain the present invention.

Accordingly, as seen in FIG. 7, an ion implantation step is performed(as part of step 425 noted earlier) of an impurity species (such asBoron) (shown as circles 1030) through sacrificial oxide layer 1018 at adose of approximately 2 to 3*10¹⁴ atoms/cm². For reasons set out in theprior King et al. applications, it is preferable to introduce chargetraps at or near an interface of substrate 1000, in those areas 1015where an NDR element is to be formed. This can be accomplished by one ofseveral known approaches, including ion implantation and/or diffusion ofan appropriate species, or deposition of a trap-containing dielectriclayer.

While Boron introduced by an implant is preferably used herein, otherelemental species may be used as charge traps as well, includingsilicon, indium, arsenic, phosphorus, antimony, fluorine, chlorine,germanium, or a metallic species. In some instances it may be possibleform traps using water (from a steam ambient) as well. Other mechanismsfor introducing the impurities can also be used, such as deposition of alayer of material containing the charge traps or charge-trappingspecies. For example, a doped silicon film can be deposited and oxidizedto form an oxide film containing a high density of charge traps.

An advantage of the present invention is that the onset of NDR behaviorcan be controlled through selecting a target trap energy level. In turn,the trap energy level can be engineered through suitable process controlparameters such as through selection of a particular impurity speciesand/or trapping layer dielectric.

A mask can be used to selectively form the charge trapping region inthose areas 1015 where an NDR element is to be formed, and in someinstances so that it does not extend across an entire region 1015 ofsubstrate 1000, but is instead limited to some smaller areacorresponding to a later gate region of an NDR FET, or even a limitedportion of such gate region. In some cases, for example, it may bedesirable to form a trapping region only near a source region, or onlynear a drain region, depending on the expected device biasing andoperational characteristics. To maximize “source side” trapping, forexample, charge traps can be selectively arranged to extend from asource region, and not extend entirely through the channel to a drainside. A variable distribution of traps might be employed along a lengthof the channel so as to effectuate a trapping rate that variescorrespondingly and results in a faster switching speed.

It is expected that routine experimentation will yield a variety of trapdistributions for optimizing different characteristics of an NDR FET,such as switching speed, V_(NDR), noise immunity, leakage, subthresholdswing, V_(t), etc. Thus it will be understood by those skilled in theart that while it is shown as extending throughout all of region 1015,the invention is not limited to such implementations, and in fact avariety of charge trapping structures may be used advantageously fordifferent applications.

Thus, the present detailed description continues with a discussion ofFIG. 8, which is a schematic cross-sectional view showing the step offorming an initial insulating layer (after sacrificial oxide layer 1018is removed) on the surface of the substrate in a first region 1015 wherean NDR FET of a preferred embodiment is to be formed as part of step 425described above. This initial insulating layer 1020 functions as part ofa gate insulator for a to-be-formed NDR FET, and can also serve as acharge trapping region for such NDR FET. It is formed on the surface ofsubstrate 1000 in active areas 1015 by one of several well-knowntechniques, including thermal oxidation of silicon. Physical vapordeposition and chemical vapor deposition can also be used. Thiselectrically insulating layer 1020 can consist entirely or in part ofSiO₂, SiO_(x)N_(y), Si₃N₄, or a high-permittivity dielectric materialsuch as metal oxide or metal silicate or their laminates, or, of course,as a combination of one or more different material layers.

As with other processing steps noted herein, an advantage of the presentinvention lies in the fact that this layer (as patterned later) can beshared by both conventional and NDR FET devices. Alternatively viewed,from a process integration impact, the existence of such layer innon-NDR regions during these NDR FET formational steps does notnegatively impact the structure, performance or reliability of anynon-NDR elements. Nonetheless, in some applications it may be desirableto mask and etch layer 1020 in those areas where non-NDR elements are tobe formed, so that charge trapping regions are not formed later acrossall regions of the substrate.

In an alternate embodiment, traps are formed by directly implanting thegate insulator layer 1020 using a combination of energies and speciesthat ensure a high concentration at a channel interface and a lowconcentration in a bulk region of layer 1020.

In yet other embodiment, multiple charge trap formation steps could beemployed, either as part of a standard process for making a single NDRdevice, part of a fine-tuning process, or even part of a standardprocess for making different kinds of NDR devices on the same substrate.For example, some traps could be introduced in the channel region beforethe gate insulator layer 1020, and some could be introduced after toachieve a target trap profile, including trap energy, trap concentrationand trap distribution. The two different sets of traps could also bedifferent impurities and/or implant species if it is desired to havemultiple trap profiles, such as different trap energies to trapdifferent types of charge carriers, or different trap types whichtrap/de-trap at different rates. In the case where different NDR devicesare being made at the same time on a substrate, appropriate maskingsteps could be used to ensure that any additional subsequent trapformation operations are only performed for selected NDR devices.

FIG. 9 is a schematic cross-sectional view showing the step ofselectively introducing a second type of impurity (at least in regions1015 where an NDR-FET of a preferred embodiment is to be formed) havingan opposite conductivity to Boron as part of a counter-doping step 430noted above. In a preferred approach, this second type of impurity isArsenic (shown with an “x” 1031 in FIG. 9) implanted at a concentrationof about 1*10¹⁴ atoms/cm² and at relatively low energy. This step hasthe effect of lowering a net p-type concentration later in a surfaceregion of the channel of an NDR FET. This leads to improvements in boththreshold (V_(t)) and sub-threshold swing (S) characteristics. Inparticular, a V_(t) of an NDR FET can be reduced, and a steepsubthreshold swing can also be realized, both factors which are criticalfor ensuring proper scaling performance in subsequent generations ofsubmicron devices. These improvements can also be exploited in the formof lower gate bias voltages and larger PVRs for integrated circuitapplications using the present inventions.

After the implantation step(s) (for traps and/or counter-doping) arecompleted, a thermal annealing step (corresponding to step 435 in FIG.4) is preferably performed to reduce implantation-induced damage. Thiscan be done in an inert ambient (Ar or N₂) or an oxidizing ambient (O₂or H₂O) for a predetermined time (e.g., several hours) at apredetermined temperature (e.g., 550° C.). Other techniques (e.g. RTA),temperatures, and times will be apparent to skilled artisans from thepresent teachings and from routine experimentation for any particularimplementation. The purpose of this step is to further ensure that thetrap distribution will be concentrated at an interface with the channel,rather than within a bulk region of trapping layer 1020.

In the absence of an anneal step, for example, Boron may undesirablydiffuse rapidly with the aid of point defects into a bulk region of thetrapping layer, resulting in a high level of gate leakage current. It ispreferable to have a high concentration of traps at achannel/gate-insulator interface, and a relatively low concentration ina bulk region of the gate insulator. These concentrations shouldpreferably be at least two or three orders of magnitude in differencemeasured in terms of atoms per cubic centimeter. By keeping the trappingsites in this region (i.e., within about 0.5 nm of the channelinterface) gate leakage current is further minimized. The size of thisregion will vary, of course, from geometry to geometry for anyparticular generation of process technologies.

Other generally accepted techniques for reducing such implant damagethat are known in the art (at this time or later developed) will also beequally useable with the present invention. Again, it will be understoodby those skilled in the art that a trap formation process that does notuse an implant, or does not result in excessive trap sites in the bulkof the gate region, will not necessarily require such an annealing step.For example, as discussed herein, if the traps are implanted (placed)directly through the gate layer at a later time, their distribution canbe concentrated in a particular region through a suitable selection ofenergies. Alternatively, a composite gate oxide can be used (i.e., animplant, a thermal oxidation, and then a deposition; or a deposition, animplant, and then a thermal oxidation) to incorporate the traps at aninterface using a thermal cycle instead. Further variations will beapparent to those skilled in the art from the present teachings.

In any event, at least in those implementations where trapping layer1020 is formed over the entire substrate, it is then selectively removed(not shown) from the areas where conventional FETs are to be formed(region 1015′), and from any other areas (including in region 1015)where it is not needed/desired.

FIG. 10 is a schematic cross-sectional view showing the step of formingan additional insulating layer 1040 on substrate 1000 to serve as a highquality gate insulator for both NDR FET and other conventional FETs(corresponding to step 440 in FIG. 4). Gate insulating film 1040 can beformed by one of several techniques, including physical vapor depositionand chemical vapor deposition. Gate insulating film 1040 can consistentirely or in part of SiO₂, SiO_(x)N_(y), Si₃N₄, combinations of thesame, or a high-permittivity dielectric material such as metal oxide ormetal silicate or their laminates.

If the gate insulating layer 1040 is formed by thermal oxidation, thenit may be located beneath layer 1020, and may be thinner in the areaswhere NDR FETs are to be formed (region 1015) than in other areas(including in region 1015′). In this case, the layer 1040 will serve asthe charge trapping layer rather than as a high-quality gate insulator,with charge traps formed via the incorporation of impurity speciesduring the thermal oxidation process or subsequent process steps.

It should be noted that additional layer 1040 is unnecessary in thosecases where conventional FETs are not being made at the same time,because a single oxide layer can be grown with sufficient thickness ofcourse as part of layer 1020. Nonetheless, a composite gate is preferredin mixed embodiments of NDR and non-NDR FET elements to accommodate theneed for additional gate insulators in the latter devices.

After the gate insulator is formed, an additional thermal annealingoperation (corresponding to step 445 in FIG. 4) is preferably performedto further optimize a distribution of the charge traps—i.e. increasetheir concentration at a channel/gate-insulator interface. Thisoperation is preferably performed with a rapid thermal anneal (RTA) at1100° C. for a short time—i.e. between 1 and 10 minutes. Othertemperatures and times will be apparent to skilled artisans from thepresent teachings and from routine experimentation for any particularimplementation. The inventor has further determined that an RTAoperation is superior to a conventional furnace operation (i.e., 1 hourat 1000° C. in a N₂ ambient) in terms of enhancing a distribution oftrapping sites near the Si/SiO₂ interface.

As the distribution of trapping sites affects the ultimatepeak-to-valley ratio (PVR) of the NDR device of the present invention,selection/control of this process step can be exploited to set such PVRto a target value. In other words, different applications requiringdifferent PVRs could be manufactured by simply adjusting a time ortemperature of an RTA, or by selecting an RTA operation over a furnaceoperation to increase a PVR value.

FIG. 11 is a schematic cross-sectional view showing the step ofdepositing a gate electrode layer 1050 for both NDR FETs andconventional FETs. The gate electrode material 1050 may bepolycrystalline silicon (poly-Si) or a silicon-germanium alloy(poly-SiGe), or it may be a metal or metal alloy or conductive metalnitride or conductive metal oxide. An advantage of the presentinvention, again, is apparent because the gates of both NDR FETs andconventional FETs can be made of the same material, and formed at thesame time.

If gate electrode material 1050 is poly-Si or poly-SiGe, it may be dopedin-situ during the deposition process or it may be doped ex-situ by ionimplantation and/or diffusion, to achieve low resistivity and a properwork function value. The final gate electrode also may consist of amulti-layered stack, with a lowest layer providing a desired gate workfunction and overlying layer(s) providing sufficient thickness andconductivity.

The gate electrode layer 1050 is then patterned using standardlithography and etching processes to form multi-layer gate electrodes1060 and 1060′ (FIG. 12) which corresponds to step 450 (FIG. 4). At thispoint, an optional post-gate-etch re-oxidation anneal operation (step455 in FIG. 4) is performed in some instances to heal any damage to thegate insulator along the edges of the gate electrodes and possibly tofurther enhance a concentration (or formation) of charge traps.

While a steam anneal can be used (e.g., 10 minutes at 750° C. in steamambient, followed by 1 minute at 1050° C. in N₂) for some embodiments,the beneficial aspects of such approach are not uniform across allimplementations. In other words, while some thinner (i.e., 5.5 nm) gateinsulator applications may benefit from such operation, other relativelythicker gate (i.e., 7 nm) insulator applications may not. This isbecause it is believed that while the steam may assist in forming newwater based traps near an Si/SiO₂ interface, the temperature exposurealso serves to counter-act this effect by driving some of thetrap-associated impurity atoms away from such interface into a bulkregion. When the gate is relatively thick, this results in a greatermigration/dilution of the trap concentration near the interface, thusresulting in reduced performance. Thus, the inventor believes that aconventional post-gate reoxidation anneal may be more useful for thinnergate oxides. Nonetheless, any comparable annealing mechanism that bothcreates new traps and yet minimizes diffusion of existing traps couldalso be employed for either application (thin or thick gate insulators).

FIG. 13 is a schematic cross-sectional view depicting a simplifiedexplanation of the resulting effects of one or more annealing steps,which, as noted above, are used to increase a density of charge traps1037 at a channel interface of the NDR FET of a preferred embodiment. Itwill be understood that this figure, and many elements therein—thetraps, the trap location, etc., are not drawn to scale, and that thedepiction is merely intended as an instructive tool for comprehendingthe present teachings.

FIG. 14 is a schematic cross-sectional view showing the step of forminglightly doped source/drain regions corresponding to step 460 in FIG. 4.In a preferred embodiment, an n-type dopant such as Arsenic (shown withan * symbol) is implanted with an energy of 10 keV and a dosage of3*10¹⁵ atoms/cm². The inventor has determined that Arsenic is superiorto Phosphorus in terms of achieving a higher overall PVR for an NDRdevice of the present invention. While the reasons for this are notentirely clear, it is believed that As diffuses more slowly than P sothat a higher doping concentration can be achieved with the former. Thisin turn results in a higher peak electric field in a drain region of thechannel, creating more energetic electrons, and thus more chargetrapping. A lower V_(NDR) can be achieved for similar reasons.

Accordingly, a desired PVR value can also be controlled to some extentfor an NDR device through suitable selection of an LDD dopant species,energy, etc. It should be noted that the shallow source/drain extensionregions may be formed in the NDR-FET areas 1015 simultaneously with theshallow source/drain extension regions in the IGFET areas 1015′. Thedopant concentration and junction depth of the shallow source/drainextensions for the NDR-FET can be made to be the same, or different fromthose for the NDR-FET, if necessary, by selective (masked) ionimplantation. Furthermore, in some embodiments, it may be desirable toform the shallow source/drain regions after the heavily dopedsource/drain regions described below.

A conventional anneal operation may be performed after the LDD implant(as noted in step 465) to anneal out any damage, and further control atarget PVR.

FIG. 15 is a schematic cross-sectional view showing the step of formingmore heavily doped drain/source regions 1070 and 1071 for an NDR FET andother conventional FETs (as noted in steps 470–475). In this case, deepsource and drain regions are offset from the edges of the gate electrodeby spacers 1025 formed along the sidewalls of the gate electrodes. Thesidewall spacers are formed by conformal deposition and anisotropicetching of a spacer film in conventional fashion. The thickness of thisspacer film determines the width of the sidewall spacers and hence theoffset from the gate electrode. A variety of such spacer techniques areknown in the art and can be used with the present invention. Again,preferably, sidewall spacers are formed at the same time for both NDRand non-NDR FETs.

Source and drain regions (step 475 in FIG. 4) 1070 and 1071 are formedby ion implantation of n-type dopants such as arsenic and/or phosphorusand subsequent thermal annealing (step 480) using conventionaltechniques to remove damage and to activate the dopants. In thisparticular implementation, gate electrodes 1060 are sufficiently thickto prevent implanted ions from entering the surface of substrate 1000underneath the gate electrodes.

As shown in a simplified perspective in FIG. 16, device fabrication iscompleted (steps 485, 490 and 495 in FIG. 4) by formation of silicide1085, 1080 on the surfaces of the source and drain contact regions andpossibly the gate electrode to provide low-resistancemetal-to-semiconductor contacts, followed by deposition of one or moreelectrically insulating interlayer films 1075, 1077, formation ofcontact holes and filling of these holes with metal plugs 1081, 1086,deposition and patterning of one or more metal layers 1083 and 1087 toform interconnections, and a low-temperature (350° C.–450° C.) anneal ina hydrogen-containing or deuterium-containing ambient (forming gas).

Multiple layers of metal wiring, if necessary, may be formed bydeposition and patterning of alternate layers of insulating material andmetal. It will be understood that the silicide contacts 1080 and 1085may be formed of low resistivity phases of titanium silicide, molybdenumsilicide, cobalt silicide, or nickel silicide compounds, and may beconnected to only one of the gate or source/drain regions depending onthe particular application. The plugs 1081 and 1086 may be formed ofTungsten, Aluminum, Copper or other metallic materials. Insulating films1075 and 1077 may be CVD films, spin-on glass, and/or any other acceptedinsulating material, including air gaps. Metal interconnect layers 1083,1087 may be Aluminum, Copper, or some other low resistivity metal.

In this manner, a semiconductor device comprising one or more IGFETelements and one or more NDR-FET elements can be manufactured on acommon substrate utilizing a fabrication sequence utilizing conventionalprocessing techniques. Those skilled in the art, of course, willappreciate that the aforementioned steps might be useful in otherprocessing environments as well, including for manufacturing other NDRdevices such as silicon based resonant tunneling diodes, two-terminalNDR FETs adapted as diodes, thyristors, etc.

While not shown explicitly, an NDR FET and a conventional IGFET have anumber of regions that are formed from common layers that are laterpatterned, including: a common substrate 1000; a gate insulator film1040 and 1040′; a conductive gate electrode layer 1060 and 1060′;interlayer insulation layers 1075 and 1077; metal plugs/layers 1081,1083 and 1086 and 1087. Furthermore, they also share certain isolationareas 1010, and have source/drain regions 1070, 1071 and 1070′, 1071′formed at the same time with common implantation/anneal steps.

In some cases, there can be direct sharing of such regions of course, sothat the drain of an NDR FET can correspond to a drain/source of anIGFET, or vice versa. Regions can be shared, of course, with twoterminal NDR FETs adapted as diodes, as well. It will be understood thatother processing steps and/or layers may be performed in addition tothose shown above, and these examples are provided merely to illustratethe teachings of the present inventions. For example, additionalinterconnect and/or insulation layers are typically used in ICs and canalso be shared.

Experimental Data Results

Experimental NDR FET devices with drawn gate lengths down to 125 nm werefabricated with the following basic parameters: 7 nm gate oxidethickness; 2×10¹⁴ cm⁻² channel implant dose; 1100° C.post-gate-oxidation RTA anneal; 3×10¹⁵ cm⁻² arsenic-doped LDD.

It should be noted right away that this prototyping process is notidentical to the preferred process described earlier. For example, nothermal anneal was performed before a gate oxide was deposited. Nor wasa counter-doping implant performed in the channel (e.g., of As), tolower the V_(t) and subthreshold swing. A single layer of gateinsulating material was used. Thus, this prototyping process wasintentionally designed and primarily crafted for purposes oftesting/characterizing the expected behavior and performance of NDRdevices, and verifying their scalability and suitability forconventional MOS circuit applications. Consequently, the resultsobtained are not necessarily reflective of the actual results that wouldbe obtained for a commercial production, or for any particular actualimplementation of the present invention in a particular channelgeometry, within a particular fabrication facility, using a particularset of design rules, or a using a particular set of processingequipment.

Nonetheless the inventor submits that these test results are useful forillustrating a number of basic key features and advantages of thepresent invention. Furthermore, they serve to further validate the basicoperational features of the invention, including a FET with switchablenegative differential resistance.

Dependences on Gate Bias and Gate Length

The dependences of NDR FET current-vs.-voltage (I–V) characteristics ongate bias and gate length were measured. FIG. 17A shows how thetransistor current varies with gate bias. Fairly typical behavior isobserved for drain biases below V_(NDR), with the transistor currentincreasing˜linearly with increasing gate drive V_(gs)−V_(t). For drainbiases above V_(NDR), the current decreases exponentially withincreasing V_(d). The valley current increases with increasing gatedrive, but not as rapidly as the peak current.

FIGS. 17B and 17C show how the peak current and valley currentrespectively vary with gate bias and gate length.

In FIG. 17B it can be seen that a peak drain current increases withincreasing gate drive and also with decreasing gate length, as expected.

In FIG. 17C it can be seen that a valley drain current also increaseswith increasing gate drive, which is reasonable. However, the valleydrain current decreases with decreasing gate length. This is alsoreasonable, because energetic carriers (generated at a drain end of thechannel) are trapped at high drain biases to effect an increase in Vt.As the gate length is decreased, these carriers are trapped closer tothe source end of the channel and hence they increase the transistor Vtmore effectively.

In FIG. 17C it can be seen that a valley drain current also increaseswith increasing gate drive, which is reasonable. However, the valleydrain current decreases with decreasing gate length. This is alsoreasonable, because energetic carriers (generated at a drain end of thechannel) are trapped at high drain biases to effect an increase in Vt.As the gate length is decreased, these carriers are trapped closer tothe source end of the channel and hence they increase the transistor Vtmore effectively.

As seen in FIG. 17D, the net effect of a decrease in gate length is asignificant increase in the peak-to-valley ratio. It should be notedthat at high drain biases, reverse-bias pn-junction breakdown current isa significant component of the drain current because of the relativelyhigh level of doping in the channel. Thus, to see the true valleycurrent of the NDR transistor, the source current must be monitored. Thedependence of the PVR for source current is plotted in FIG. 17E. The PVRincreases to ˜100 as the gate length is scaled down to 125 nm.Consequently, as can be seen in this test data, NDR embodiments of thepresent invention are extremely scalable, thus ensuring their utility infuture deep submicron silicon processing technologies.

Ideally, the valley current of the present NDR device should comparequite favorably with the off-state leakage current of a conventionalMOSFET. In the present NDR FET device in fact, the off-current can becontrolled quite effectively (and differently than a current state ofthe art FET) by the areal trap density N_(T) (number of traps per unitarea)

Temperature Dependence Data

The present invention is expected from a theoretical perspective to showtemperature performance superior to other NDR alternatives, because,among other things, the average kinetic energy of an electron is higherat elevated temperatures. Thus, the trapping and de-trapping rates canbe expected to increase, i.e. the response time of the NDR-FET shouldimprove with increasing temperature. However, since the mean free pathof an electron in the channel will decrease, it is conceivable thathigher electric fields may be needed to generate electrons which areenergetic enough to cause the NDR behavior. The latter can be achieved,of course, in any number of ways previously described.

Additional temperature dependence data for one embodiment of an NDRdevice is thus illustrated in FIG. 17F. Again, while this device wasconstructed as a test vehicle, it demonstrates certain operationalbehaviors of various embodiments of the present invention, including thefact that an overall PVR value is substantially constant over a widetemperature range of 25° C. to 125° C. This is because, as can be seenin the figures, while a peak current increases with temperature, avalley current also increases. Accordingly, some embodiments of thepresent invention can be tailored to operate with relative temperatureindependence over a reasonably wide temperature range.

As can be seen in the graphs of FIG. 17F both the peak current andvalley current increase slightly as the temperature increases to 125° C.The annotations on this graph include lines corresponding to a draincurrent I_(d), and symbols corresponding to a source current I_(s). Thesolid symbols and line are for 25° C. measurement; open symbols anddashed line are for 125° C. measurement.

The peak current increases by about 20%, while the valley currentincreases by a factor of ˜3 over the entire temperature range; this isrelatively small compared to a conventional MOSFET, in which the leakagecurrent increases exponentially with temperature. Overall, however, theNDR-FET peak-to-valley current ratio (the key performance metric for aNDR device) remains fairly constant over a wide range of temperatures.

Hence, the NDR FET of the present invention can clearly meet theoperating temperature specifications for commercial IC products. Infact, it is expected that optimized embodiments of the present inventionusing the aforementioned preferred processes described above can achievea PVR in excess of 10⁶ across a very wide temperature range, making themparticularly suitable for military, aerospace, automotive, and similartemperature demanding environments. This feature, in addition to itscompatibility with a conventional CMOS process, makes the NDR-FET standsout among all known NDR devices in its promise for high density ICapplications.

It should be noted that prior-art NDR devices such as the tunnel diode,resonant tunneling diode, thyristor, real-space transfer transistors,etc. show significantly degraded performance at elevated temperatures.For instance, a thyristor-based memory must operate with a relativelyhigh (>1 nA) holding current in order to guarantee stable operation at75° C. A so-called single transistor (DRAM-based) SRAM will havesignificant power consumption at elevated operating temperatures becausehigher refresh rates must be used to compensate for higherpass-transistor leakage.

PVR & V_(NDR) Control Through Various Process Parameters

The effects of various process parameters on PVR and V_(NDR)characteristics were also examined. This was done by examining PVR andV_(NDR) values for various experimental splits which yielded workingdevices. Thus, as seen in FIG. 17G, the 7 nm gate oxide NDR test devicewafer which yielded the results of FIGS. 17A to 17F is designated asW#A3, corresponding to Wafer A3. Additional wafer prototypes were alsotested with various processing variations, including:

-   (1) different channel implant dosages for forming the traps (i.e.,    Boron at 2*10¹⁴ or 3*10¹⁴ atoms/cm²);-   (2) different lightly doped drain species (P⁺ or As⁺) and dosages;-   (3) different post-gate-oxidation annealing conditions (RTA or    furnace)-   (4) different steam re-oxidation conditions-   (5) different gate insulator thicknesses

PVR and V_(NDR) values are summarized in FIG. 17G and FIG. 17H,respectively, for NDR FETs with drawn gate length 180 nm; drain currentvalues are noted with hashed bars, while source current values are shownwith solid bars. Several key observations can be made from this testdata, which is extremely useful from the perspective of effectuatingprecise PVR and/or V_(NDR) control for a particular embodiment of an NDRdevice. In particular, it can be seen that a desired target PVR/V_(NDR)value can be obtained by fine tuning one or more standard processoperations during a manufacturing process. This allows for a widevariety of PVR and/or V_(NDR) values, and further ensures thatpredictable, reliable yields and results can be obtained for an NDRprocess.

In a preferred embodiment, V_(NDR) is set to be slightly lower thanone-half the power-supply voltage V_(dd), i.e. V_(NDR)<=V_(dd)/2.Nonetheless, different V_(NDR)s can be achieved at different areas of asemiconductor substrate through appropriate process controls asdisclosed herein.

Thus, as the test data shows, as a result of the unique structure andoperational features of the present invention, a desired PVR and/orV_(NDR) characteristic is easily set and controlled within aconventional MOS manufacturing facility using one or more conventionalprocessing operations. This ease of manufacturability ensures thatappropriate target values for PVR and V_(NDR) can be achieved for a widevariety of target applications. While the present disclosure provides anumber of examples of process variations which can be used to control aPVR and V_(NDR) behavior, other examples will be apparent to skilledartisans from the present teachings. Thus, the present invention is byno means limited to any single variant, or combinations of variants ofsuch PVR and/or V_(NDR) process control techniques.

PVR and V_(NDR) Control Through Channel Implant Dose Control

FIG. 17G shows that higher PVR values are achieved with higher boronimplant dose. This is expected because the density of traps iscorrelated with the concentration of boron incorporated into the oxidenear the Si/SiO₂ interface. As noted earlier, however, the concentrationof traps should not be made too high, in order to avoid trap-to-trapconduction.

FIG. 17H shows that V_(NDR) is slightly lower for higher boron implantdose because of the higher average vertical electric field in theinversion channel. (Larger values of V_(g) are required to achieve 1 Vgate drive, because V_(t) is larger.) For a larger vertical electricfield, the lateral electric field (hence V_(d)) does not need to be ashigh in order to create the hot electrons which can be trapped.

Accordingly, a desired or target PVR/V_(NDR) value can also beeffectuated by controlling the type of implant/dosage used in anyparticular manufacturing environment.

PVR and V_(NDR) Control Through Post-Gate-Oxidation Anneal

As seen in FIG. 17G, significantly higher PVR values are achieved withan 1100° C. RTA as compared with the 1000° C. furnace anneal. Thisindicates that a higher density of traps at the Si/SiO₂ interface isachieved with an 1100° C. RTA. Thus a desired PVR value can also beeffectuated by controlling the type of thermal annealing step performedin any particular manufacturing environment.

At this time, the experimental data (as seen in FIG. 17H) does not showthat V_(NDR) has a strong dependence on post-gate-oxidation annealingconditions.

PVR and V_(NDR) Control Through LDD Implant Dose

Significantly higher PVR values are obtained with As-doped LDD ascompared with P-doped LDD as seen in FIG. 17G. The inventor believesthis is the case because As diffuses more slowly than P so that a higherLDD doping concentration is achieved with As. This in turn provides ahigher peak electric field in the drain region of the channel, hencehotter electrons and more charge trapping. Apparently, in theexperimental wafers, the LDD ion implantation damage was not completelyannealed out for the higher dose (3×10¹⁵ cm⁻²) As implant, whichresulted in lower peak current and higher valley current and hencedegraded PVR. Again this can be corrected using known annealingtechniques.

In FIG. 17H, it can be seen that V_(NDR) is lower for As-doped LDD ascompared with P-doped LDD. This is because the drain bias required toachieve the critical peak electric field in the drain region of thechannel is lower (due to the higher LDD doping concentration).

Consequently, an LDD operation provides yet another mechanism forsetting or fine-tuning a desired PVR/V_(NDR) value using conventionalMOS process operations.

PVR and V_(NDR) Control Through Gate-Oxide Thickness

FIG. 17J and FIG. 17K show similar test data for PVR and V_(NDR),respectively, except for a slightly thinner gate oxide (5.5 nm). Thisdata is also useful because it illustrates yet another tool availablefor process designers to effectuate a variable PVR value. Namely, asseen in this figure, for all other parameters being equal, an overallPVR value is lower than for comparable NDR devices having a 7 nm gateoxide.

Accordingly, higher PVR values are also achieved with thicker gateoxide. This is expected because a given density of charge traps (N_(T))will effect a larger increase in V_(t) for a thicker gate dielectric:ΔV _(t) ≈q*N _(T) /C _(ox)

As an example, for N_(T)=5×10¹²/cm² and 7 nm SiO₂ gate dielectric,V_(t)≈1.6 V, so that a “peak to valley ratio” (PVR) close to 10⁶ shouldbe attainable (assuming V_(gs)−V_(t)=1V and S is about 100 mV/dec). Theeffective PVR also can be enhanced (by up to 100×) by dynamicallyvarying the gate bias to either enhance the peak current and/or to lowerthe valley current. This type of in-circuit PVR adjustment, duringoperation of an NDR device, is another benefit of the present inventionsthat can be used in some embodiments.

In FIG. 17K, it can be seen that V_(NDR) is slightly lower for a thickergate oxide because of the higher average vertical electric field in theinversion channel. (Larger values of V_(g) are required to achieve 1 Vgate drive, because V_(t) is larger.) For a larger vertical electricfield, the lateral electric field (hence V_(d)) does not need to be ashigh in order to create the energetic electrons which can be trapped.

For these reasons, a desired PVR/V_(NDR) value can also be effectuatedby controlling the type and thickness of a gate insulator used in anyparticular manufacturing environment.

PVR and V_(NDR) Control Through Steam Anneal

The effect of the steam anneal cannot be clearly ascertained from theexperimental results. As seen in FIG. 17J, for a relatively thin gateoxide (5.5 nm), the PVR is consistently higher if a steam anneal wasemployed. As seen in FIG. 17G, however, for thick gate oxide, however,the PVR is marginally (but consistently) lower if a steam anneal wasemployed.

V_(NDR) is generally lower in all cases if a steam anneal was employed.

These results suggest that, as noted earlier, a steam anneal is helpfulfor forming additional charge traps near the Si/SiO₂ interface. However,in some cases it also enhances boron diffusion away from the interface(and thereby lowers the trap-state density at the interface) if the gateoxide is thick.

Accordingly, it appears that for some geometries, a desired PVR/VNDRvalue can also be effectuated by using a steam anneal process tomanufacture an NDR device.

NDR FET Reliability

In the NDR FET, carriers tunnel through an ultra-thin interfacial oxideinto and out of traps when V_(ds)>V_(NDR). The vast majority of thesecarriers will not have sufficient kinetic energy to cause new traps tobe formed in the “tunnel oxide”. Even if new traps were to be formed inthe “tunnel oxide” (e.g. by high-energy electrons in the tail region ofthe electron energy distribution), they would likely serve to enhancethe speed of the NDR FET, because these new traps would be formed closerto the Si/SiO₂ interface than the original traps.

Although reliability issues for the NDR FET were not tested explicitly,the inventor believes that the existing body of knowledge on SiO₂ pointsto the fact that such devices should be as good or better thanconventional MOSFETs. Based on the trend of increasingcharge-to-breakdown Q_(BD) (to infinity as oxide thickness decreases tozero) with decreasing oxide thickness, it is reasonable to expect thatthe “cycle-ability” of the NDR FET will be very high (e.g. >>10¹² cyclesbetween high-V_(t) and low-V_(t) states).

It is known that conventional hot carriers in the channel (i.e., >3.1eV) are responsible for degradation in MOSFET performance, because ofthe damage which they cause to the oxide interface as well as in thebulk of the oxide. The NDR FET in fact should provide superior results,because in such device, the amount of hot carriers is limited becauseonly energetic carriers are generated (i.e. about 0.5 eV) and thetransistor turns itself off at high V_(ds). The energetic electronswhich tunnel into the traps embedded within the oxide are generally not“hot” enough to cause damage. Thus, the inventor expects the NDR FET tohave reasonably good reliability in commercial applications.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. It will be clearly understood by those skilled in theart that foregoing description is merely by way of example and is not alimitation on the scope of the invention, which may be utilized in manytypes of integrated circuits made with conventional processingtechnologies. Various modifications and combinations of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thedescription. Such modifications and combinations, of course, may useother features that are already known in lieu of or in addition to whatis disclosed herein. It is expected, given the unique characteristics ofthe inventive device and methods (which permit a variety ofmanifestations), and the rapid progress in the arts of this field, thatadditional embodiments utilizing different yet-to-be developedmaterials, structures and processes will most certainly be developedbased on the present teachings.

It is therefore intended that the appended claims encompass any suchmodifications, improvements and future embodiments. While such claimshave been formulated based on the particular embodiments describedherein, it should be apparent the scope of the disclosure herein alsoapplies to any novel and non-obvious feature (or combination thereof)disclosed explicitly or implicitly to one of skill in the art,regardless of whether such relates to the claims as provided below, andwhether or not it solves and/or mitigates all of the same technicalproblems described above. Finally, the applicant further reserves theright to pursue new and/or additional claims directed to any such noveland non-obvious features during the prosecution of the presentapplication (and/or any related applications).

1. A semiconductor structure comprising: a semiconductor substrate; anda dielectric layer located on said semiconductor substrate, such that aninterface region is formed between semiconductor substrate and saiddielectric layer; a channel located proximate to said interface region,which channel is capable of carrying a current that varies from a firstcurrent value associated with a conducting condition, to a secondcurrent value associated with a non-conducting condition, said secondcurrent value being substantially less than said first current value; aplurality of carrier trapping sites within said interface region, saidcarrier trapping sites being configured for trapping carriers that areelectrically biased by an electrical control field to move from saidchannel into said interface region; wherein said trapping sites arecharacterized by an energy level that is higher than a conduction bandof said channel and lower than a conduction band of said dielectriclayer and said trappings sites have a concentration and arrangement sothat said current in said channel can be varied between said firstcurrent value and said second current value by the action of saidtrapping sites adjusting said current in accordance with a value of saidelectrical control field; further wherein a ratio of said first currentvalue to said second current value exceeds ten (10) over a temperaturerange greater than 0° C. and spanning at least 50° C.
 2. Thesemiconductor structure of claim 1 wherein said ratio is substantiallyconstant over a temperature range of 100 C.
 3. The semiconductorstructure of claim 2 wherein said ratio exceeds 10³ over said entiretemperature range.
 4. The semiconductor structure of claim 1 whereinsaid ratio varies by a factor of 5 over said temperature range.